Those in the wide ranging materials joining industries have recognized the benefits of friction stir welding (FSW) since its invention, only to be precluded from widespread application due to a number of factors. FSW is a relatively simple method of solid phase welding developed by The Welding Institute in the early 1990's. The conventional process utilizes a specially shaped nonconsumable cylindrical tool with a profiled pin, often threaded, extending from a shoulder of the tool, that is rotated and plunged into a joint formed by abutting edges of the workpieces that are to be joined until a surface of the shoulder contacts the surface of the workpieces. The rotating tool plasticizes a region of the workpieces around the pin and beneath the shoulder. The tool is then advanced along the joint. The rotation of the tool develops frictional heating of the workpieces, from both shoulder friction and pin friction, as well as adiabatic heating, and the tool forces plasticized workpiece material from the leading edge of the tool to the rear of the tool where it consolidates and cools to form a high quality weld.
The FSW tool is generally a cylindrical piece with a shoulder face that meets a pin that projects from the shoulder face at a right angle, as illustrated in U.S. Pat. Nos. 5,460,317 and 6,029,879. In some instances, the pin actually moves in a perpendicular direction in an aperture formed in the face of the shoulder, as illustrated in U.S. Pat. Nos. 5,611,469; 5,697,544; and 6,053,391. The face of the shoulder may be formed with an upward dome that is perpendicular to the pin, as illustrated in U.S. Pat. Nos. 5,611,479; 5,697,544; and 6,053,391. The dome region and an unobstructed shoulder face to pin interface have been considered essential for the proper frictional heating of the workpiece material. Traditional thinking held that the dome region of the shoulder serves to constrain plasticized material for consolidation at the trailing edge of the FSW tool so as to prevent it from extruding out from under the sides of the tool. For example, U.S. Pat. No. 5,813,592 states at column 1, lines 42-51, that “In order to achieve a proper consolidation of the weld metal the probe bottom part (shoulder) must maintain during the whole welding operation (forward movement) in an intimate contact with [the] surface of the joined members. If the probe shoulder during this forward movement even temporarily ‘lifts’ from the surface a small amount of plasticised welding material will be expelled behind the probe thus causing occurrence of voids in the weld since there is no available material to fill the vacant space after the expelled material.” The present invention proves this long-held belief false.
Since FSW is a solid-state process, meaning there is no melting of the materials, many of the problems associated with other fusion welding methods are avoided, including solidification cracking, shrinkage, and weld pool positioning and control. Additionally, FSW minimizes distortion and residual stresses. Further, since filler materials are not used in FSW, issues associated with chemical segregation are avoided. Still further, FSW has enabled the welding of a wide range of alloys that were previously unweldable. Yet another advantage of FSW is that it does not have many of the hazards associated with other welding means such as welding fumes, radiation, high voltage, liquid metals, or arcing. Additionally, FSW generally has only three process variables to control (rotation speed, travel speed, and pressure), whereas fusion welding often has at least twice the number of process variables (purge gas, voltage, amperage, wire feed speed, travel speed, shield gas, and arc gap, just to name a few). Perhaps most importantly, the crushing, stirring, and forging of the plasticized material by the FSW tool often produces a weld that is more reliable than conventional welds and maintains material properties more closely to those of the workpiece properties, often resulting in twice the fatigue resistance found in fusion welds.
Despite all the advantages of FSW, it has only found very limited commercial application to date due to many difficulties associated therewith. One early problem associated with single-piece FSW tools 90, as seen in FIG. 1, was that they leave an exit hole 80 in the weld 40, as seen in FIG. 5, that must be filled after completion of the friction stir weld. Such single-piece FSW tools 90 are also plagued with premature breakage of the pin 92 during welding, resulting in the pin 92 being permanently lodged in the weld 40. Such breakage is often attributed to tool design that has relatively poor heat distribution and areas of high stress concentration, such as at the pin 92 to shoulder 91 interface, also known as the transition region 93, seen in FIG. 1. In an effort to eliminate exit holes 82 the retractable pin tool 95 was developed, as seen in FIG. 2. The retractable pin tool 95 essentially splits the conventional shouldered FSW tool 90 into two separate components, namely a shoulder portion 96 that is hollow and receives the pin 97 that may extend and retract from the shoulder 96. The independent movement of the pin 97 permits the pin 97 to be gradually withdrawn from the weld 40 while the shoulder 96 remains in contact with the workpieces 10, 20, thereby eliminating the exit hole 80.
While the retractable pin tool 95 may eliminate the exit hole 82, it has several drawbacks. The retractable pin tool 95 is prone to breakage due to the high stress concentrations at the shoulder 96 to pin 97 interface. The retractable pin tool 95 is also susceptible to binding between the pin 97 and the shoulder 96 as stirred weld metal can be forced into the gap between the pin 97 and the shoulder 96.
Another problem with both conventional shouldered FSW tools 90 and retractable pin tools 95 is the overheating caused by the shoulder 91, 96. During FSW with conventional shouldered FSW tools 90, 95 the weld 40 is repeatedly subjected to the pressure and rotation of the tool shoulder 91, 96. As a conventional FSW tool 90, 95 traverses a joint 35 the material is first exposed to the leading edge of the shoulder 91, 96 that is generally exerting a downward force on the workpieces 10, 20 of several hundred pounds, often several thousand pounds, and is rotating at RPM's ranging from under 100 rpm to over 1000 rpm, while traversing the joint 35 rather slowly, generally less than ten inches per minute (IPM), depending on the materials being joined and their thickness. Taking for example a simple illustrative case of a conventional tool 90, 95 traversing a joint 35 at 6 IPM and 800 RPM, it takes 10 seconds to traverse a one inch section of the joint 35 during which 80 revolutions of the tool 90, 95 are made, resulting in 160 exposures of weld 40 to the shoulder 91, 96 (an exposure at the leading edge and the trailing edge for each revolution). Such repeated exposure to the shoulder 91, 96 results in the overheating of the weld 40 and the associated drawbacks. Prior methods and apparatus have indicated that such top surface friction heating and weld material containment contributed by the shoulder were essential to FSW. In fact, the definition of friction stir welding in most welding references includes the mention of a tool having a pin and a shoulder, thus a tool lacking a shoulder, or a shoulderless tool, as in the present invention, is a completely new concept.
Further, conventional shouldered FSW tools 90 and retractable pin tools 95 are generally ineffective at joining workpieces 10, 20 of different thickness, as seen in FIG. 6. This is due in large part to the fact that such tools 90, 95 are designed for a specific pin 92, 97 length for a particular material thickness. Such designs necessitate a unique tool for each thickness of material to be joined. The retractable pin tool 95 may reduce the number of tools needed to make welds in materials having differing thicknesses, but it too is limited in that each retractable pin tool 95 has a limited useful range established by the diameter of the shoulder. For instance, if the material is too thick or thin then under-heating or over-heating will occur. Additionally, one can easily appreciate that the pin 97 of a retractable pin tool 95 designed for use in joining ⅛″ thick sheets will be ineffective and will fail if it is simply further extended from the shoulder 96 in trying to join ½″ thick plates.
Additionally, conventional shouldered FSW tools 90 and retractable pin tools 95 cannot be used in joining workpieces having more than slight curvature. Such tools 90, 95 provide inadequate contact, also referred to as lift-off, or result in gouging of workpieces, as seen in FIG. 18. Such lift-off and gouging results in welds having reduced aesthetic qualities that often require grinding of the surface and diminish the mechanical properties of the weld.
Yet another problem associated with conventional shouldered FSW tools 90 and retractable pin tools 95 is the flow characteristics imparted on the weld material due to the transition region 93, labeled in FIG. 1, between the shoulder 91 and the pin 92. The transition region 93 in shouldered tools 90, 95 often causes dead zones and eddies in the material flow resulting in subsurface voids and lack of fusion in the weld 40. Such problems greatly limit the robustness of the conventional tools and methods, particularly on joints that vary in geometry or heat distribution due to part shape or tooling.
A friction stir weld 40 created with conventional shouldered FSW tools 90, 95 has several distinct regions, as seen in FIG. 3, where the direction of travel of the tool 90 is into the paper. First, the metal away from the immediate vicinity of the weld 40 that is not affected by the weld is known as the base metal 50. Closer to the actual weld 40 is the heat affected zone (HAZ) 60 where the material has experienced a thermal cycle that has modified the microstructure and/or mechanical properties, yet has no plastic deformation. Next, closer to the tool 90, 95 is the thermomechanically affected zone (TMAZ) 70 where the material has seen limited plastic deformation by the tool 90, 95, and the heat from the process has also exerted some influence on the material. With the exception of aluminum, most materials exhibit recrystallization throughout the TMAZ 70. Aluminum often exhibits recrystallization in only a portion of the TMAZ, often referred to as the nugget. Within the TMAZ 70 is the stir zone 75, seen in FIG. 4, having non-uniform grain structure from the violent deformation that materials in this region undergo while hot. The stir zone 75 has a shoulder region 76 and a pin region 77. The pin region 77 is that region that nas been directly exposed to the pin 92, whereas thc shoulder region 76 is the region just outside of the pin region 77 and below the shoulder 91, 96 of the tool 90, 95. The shoulder region 76 flares out further away from the pin 92, 97 near the surface of the workpiece nearest the shoulder 91, 96, due to the effects of the shoulder 91, 96. This flared-out portion of the shoulder region 76, or re-stir area, near the surface of the weld 40 is the area most commonly exposed to overheating and the associated annealing and overageing effects that reduce the weld properties.
Additionally, the design of conventional shouldered FSW tools 90, 95 is prone to excessive wear and poor heat and load distribution. These problems are largely attributable to the longstanding belief that FSW tools must have a relatively narrow pin and wide shoulder.
Accordingly, the art has needed a tool, and associated methods, that eliminate the need for a shoulder and thereby eliminate the multitude of problems associated with the shoulder. An ideal tool would be simple in design and construction; inexpensive; allow for retractability during welding thereby eliminating the exit hole; accommodate joining materials of differing thicknesses; facilitate variable penetration depth; improve weld quality by reducing internal voids and lack of fusion; and eliminate the re-stir area of the stir region. While some of the prior art devices attempted to improve the state of the art, none has achieved the unique and novel configurations and capabilities of the present invention. With these capabilities taken into consideration, the instant invention addresses many of the shortcomings of the prior art and offers significant benefits heretofore unavailable. Further, none of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.